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Focus Calibration on Crown Graphic Special

 
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Gary Clennan



Joined: 16 May 2018
Posts: 2
Location: Canada

PostPosted: Wed May 16, 2018 1:28 pm    Post subject: Focus Calibration on Crown Graphic Special Reply with quote

Hi All! New user - fist post. I recently bought a beautiful Crown Graphic Special (SN#934971) with a 135mm Schneider-Kreuznach Xenar f/4.7 lens. I am planning on trying it out this Friday but would like to confirm my focus will be accurate before I go and shoot/process a bunch of film. I set it up on a tripod last night just to get a feel for things. What I found was that the "correct" focus point via the (top mount) rangefinder was different than the focus point determined when using the ground glass. I had a look at the RF mechanism to see if the proper cam was installed but couldn't see and stamped numbers on the cam. I have no reason to suspect it was changed out but wanted to check anyway. I then moved onto the GG and found that it had the ground glass with a Fresnel in front. These were both installed correctly. From what I have read online, it seems like all CG Special versions came from the factory with a Fresnel installed. I would like to confirm this somehow. I read that the height of the "bosses" would be a good indicator but I don't know what a boss is. The stacked GG and Fresnel sit just a bit higher than the two small rectangular pieces which sit on each side of the glass. These two pieces would be the same ones which the metal retainer clips screw into. The stacked GG and Fresnel sit flush with the four raised corner "posts" in the outer corners of the general opening. I hope this makes sense. If the Fresnel was never supposed to be there, I will just remove it. I could shoot some test shots but would prefer to not waste any film... Any help would be appreciated!
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45PSS



Joined: 28 Sep 2001
Posts: 3634
Location: Mid Peninsula, Ca.

PostPosted: Wed May 16, 2018 5:53 pm    Post subject: Reply with quote

The cam has to be removed from the camera to read the numbers on it. Lens were optically measured and the cam cut accordingly. Lens are rarely their marked focal length. They can be 0.1mm more or less to 2mm to 3mm more or less. This guidebook should be of help
https://drive.google.com/file/d/0B1sPJkp-MdghM0RuVXpkZnRnbVk/view?usp=sharing

Ektalite Field Screen was standard on Graphics starting in the early 1950's.
The fresnel, if factory original, goes fresnel smooth side against the bosses (raised metal corners of the opening) then the ground side of the ground glass against the ribbed side of the fresnel. Sounds like yours is reversed.
Ektalite field screen is graflex speak for fresnel.

The mount bosses are milled to set the ground glass optically at the film plane with the fresnel in front. If the fresnel is not factory then the sandwich may need to be shimmed to get correct focus and if the fresnel is removed then the ground glass will need to be shimmed to match the rangefinder. http://www.southbristolviews.com/pics/Graphic/manual-pdf/servicemanual.pdf

934971 was made in late 1958.
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Gary Clennan



Joined: 16 May 2018
Posts: 2
Location: Canada

PostPosted: Wed May 16, 2018 6:44 pm    Post subject: Reply with quote

Thank you! I really appreciate it. I don't recall seeing these bosses underneath the GG/Fresnel stack. I am pretty sure they just sit on a flush mount which doesn't have any raised corners. I will have to double check tonight. Do you know if my model came standard with the Ektalite screen? It sounds like the only way for me to check is to do some testing with cheap film. Maybe a test shot using only GG and then one with both. Then I could compare to using the rangefinder. Both are different and I'm not sure which one to use... Thanks!
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45PSS



Joined: 28 Sep 2001
Posts: 3634
Location: Mid Peninsula, Ca.

PostPosted: Wed May 16, 2018 9:47 pm    Post subject: Reply with quote

In the service manual linked in my previous post it says to set the rangefinder infinity then the lens infinity and stops. With the cam in the rangefinder focus it on an infinity target at least 5000 feet away. Now mount the gg/fresnel sandwich as described in my previous post and check the gg focus with the lens at the infinity stops. Does the rf and gg agree? Yes all is good, go make some exposures.
NO are the rails within 1/8 inch of fully retracted? Yes - rf infinity correct, lens infinity incorrect.

Graflex was inconsistent so no definitive answer on whether a fresnel was used or not, 90%+ chance it was. The bosses may be on the Graphic (spring) back only. Being the sandwich sits flush with the outer edge of the focus frame its likely it should have one.
If your camera was a 1952 - 1955 model it might not have a fresnel.
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